The present invention relates to a high frequency power amplifier which is used in a wireless communication system such as a cellular phone and which amplifies a high frequency signal and outputs the so-amplified signal therefrom, and a technology effective if applied to an electric part in which the high frequency power amplifier is built. The present invention relates particularly to a technology for improving control sensitivity in an area of level low in output power in a wireless communication system having a detection circuit which performs detection of an output level necessary for feedback control of output power by a current detection system.
A high frequency power amplifier for amplifying a post-modulation signal has generally been built in a transmitting-side output unit in a wireless communication equipment (mobile communication apparatus) such as a cellular phone. In the conventional wireless communication equipment, the level outputted from the high frequency power amplifier or an antenna has been detected and fed back to control an amplification factor of the high frequency power amplifier according to a transmit request level sent from a control circuit such as a baseband circuit or a microprocessor (see Patent Document 1, for example). In general, the detection of the output level has heretofore been performed using a coupler or a detector circuit or the like. The detector circuit is often configured as a semiconductor integrated circuit separated from the high frequency power amplifier.
Also the coupler is a device for detecting an output level via a capacitor formed between the device and a conductor disposed in parallel with an output line (microstrip line) formed in a discrete part or an insulating substrate (module substrate). The device is larger in size than a device formed on a semiconductor chip. Incidentally, the directional coupler (coupler) has been described in, for example, “Basis of Microwave and Its Application” published by Sogo Electrics Press on Jun. 10, 1995, P185–P198. A ceramic-laminated low-pass filter and a directional coupler for mobile communications have been described in “Electronic Material” published by Institute for Industrial Research in the April issue in 1999, P91–P95.
Since semiconductor integrated circuits different from the high frequency power amplifier, and electric parts are used in large numbers in the conventional output level detection system of high frequency power amplifier, it becomes difficult to bring a module into less size. Further, when the coupler is used, a reference voltage might be applied to one end of the coupler in order to improve detection sensitivity. In such a case, a problem arises in that since there is a need to optimally set the reference voltage and make adjustments to voltages or the like corresponding to variations in part, the burden on a set maker increases. Also a problem arises in that when the coupler is used, power dissipation also develops.
Further, as a recent cellular phone, a dualband type cellular phone has been proposed which is capable of handling a signal based on a system like a DCS (Digital Cellular System) using frequencies lying in, for example, a 1710 MHz to 1785 MHz band in addition to a system called GSM (Global System for Mobile Communication) using frequencies lying in a 880 MHz to 915 MHz band. Since output power amplifiers are also provided corresponding to respective bands in a high frequency power amplification module used in such a cellular phone, there is also a need to provide couplers for detecting their output levels according to the respective bands. Therefore, a size reduction in module becomes more difficult.
Thus, the present applicant has made and filed the invention about a current detection type wireless communication system which is provided with an output detecting transistor that receives therein an input signal of a power amplifying transistor for amplifying a high frequency signal and causes a current proportional to the current flowing through the power amplifying transistor to flow, and a current mirror circuit for transferring the current of the output detecting transistor are provided, and which converts the current for a transfer destination of the current mirror circuit into its corresponding voltage and sets it as a detected signal of an output level, and compares the detected output level and a transmit request level to thereby control an output level (see Japanese Unexamined Patent Application No. 2000-523757).
[Patent Document 1]
Japanese Unexamined Patent Publication No. Hei 2000-151310